Forstmann had led IMG, the sport, fashion and media agency, since 2004, when he bought the group.
IMG is the largest sports agency in the world, representing Tiger Woods, Roger Federer, Nike and Rolex. The firm has not announced a replacement for Forstmann, who was chairman and chief executive.
Before buying IMG, Forstmann co-founded Forstmann Little & Co, which returned $15bn to investors under his leadership through investments in firms from Dr Pepper to Gulfstream Aerospace to 24 Hour Fitness Worldwide.
In the 1980s, Forstmann was also a high-profile critic of the junk bond movement on Wall Street.
And he lost a bidding war for Nabisco – a story chronicled in the book and subsequent film adaptation “Barbarians at the Gate”.
The Yale graduate was also a philanthropist, co-founding the Children’s Scholarship Fund in 1998 and acting as a trustee of the Nelson Mandela Children’s Fund.